High speed MODE I crack growth in elastic-plastic materials, involving large scale plasticity and dynamic effects connected to rapid propagation, is faced through a cohesive model to tune force nodal release. The stress resisting to the opening of the edges in the cohesive zone should account of effective stress field ahead crack tip. In this paper a reference value is accounted: it represents the maximum closing stress measured at the crack tip, where the cohesive effects begin. A bi-parametric analytical formulation of stress distribution ahead the crack tip is suggested. The bi-parametric formulation is able to extrapolate the stress at the tip whatever is the T-stress (i.e. the stress acting in the direction of fracture propagation), thus completely defining the cohesive loads.